Scanned laser response studies of metal-insulator-silicon solar cells in polycrystalline czochralski silicon

Abstract

Metal-insulator-silicon (MIS) solar cells with and without grain boundaries were fabricated using polycrystalline Czochralski silicon. Scanned laser spot methods were used to provide high resolution photocurrent and surface reflection images of these cells. Electrically active grain boundaries, revealed by suppressed photocurrent collection, did not correlate perfectly with etc-produced surface features. For the laser wavelength (6328 Å) an effective photocurrent loss of 30–40% occurred over a 16μm width grain boundaries. Comparisons of short-circuit current versus open-circuit voltage over a wide range of irradiance values were made on cells with and without grain boundaries. For a typical polycrystalline cell the air mass one photocurrent loss was 8%, whereas a 3% loss was determined for the laser excitation. The open-circuit voltage reduction was about 25mV, implying an opposing current density associated with grain boundary walls of about 50% of that for the MIS barrier. This work shows the value of combining scanned light spot methods with photovoltaic characterization using cells with and without active grain boundaries. Future extensions should allow studies of detailed grain boundary electrical behavior including procedures to modulate it.